1. Technical Field
The present invention is directed, generally, to polythiophenes, and, more particularly, to head-to-tail coupled regioregular polythiophenes, block copolymers made therefrom, and methods of forming the same.
2. Background
Conducting polymers, such as polythiophenes (PTs), represent a class of polymers that are lightweight, highly processable and exhibit relatively high environmental stability, thermal stability, and electrical conductivity. These materials can be synthetically tailored to achieve desired properties such as melting point, electrical conductivity, optical and microwave absorbance and reflectance, and electroluminescene. Compared to inorganic metals and semiconductors, electrically conductive polymers have been found to be promising candidates for numerous applications, ranging from electronic and optical devices, such as field-effect transistors, sensors, light-emitting diodes (LEDs), rechargeable batteries, smart cards, and non-linear optical materials, to medical applications, such as artificial muscles.
Due, in part, to the increased demand for employing conducting polymers into a wide range of electrical and optical equipment, efforts have been made to advance the ways in which electrically conducting polymers can be improved for even greater integration into these applications. Numerous attempts to produce electrically conductive polymers that exhibit the electronic and optical properties of semiconductors and metals and the mechanical and processing advantages of typical plastics have, thus far, yielded little success. These attempts typically employ one of two distinct methods—the formation of polymer blends, and the synthesis of block copolymers.
Techniques that incorporate blends and/or composites of conducting polymers and conventional polymers include chemical and electrochemical in situ polymerizations. These methods include mechanically mixing two or more conducting and conventional polymers to form a polymer blend. Blending methods are relatively simple and cost effective when compared to methods that produce block copolymers, and can be found in various publications, such as, for example, H. L. Wang, L. Toppare, J. E. Fernandez, Macromolecules, 23, 1053 (1990); K. Koga, S. Yamasaki, K. Narimatsu, M. Takayanagi, Polym. J. 1989, 21(9), 733 (1989); Synthetic Metals, 21, 41 (1989); Synthetic Metals, 28, c435 (1989); Synthetic Metals, 37, 145 (1990); Synthetic Metals, 37, 195 (1990); Macromolecules, 25, 3284 (1992); Synthetic Metals, 22, 53 (1987); Macromolecules, 22, 1964 (1989); Polymer, 39, 1992 (1989): and U.S. Pat. Nos. 5,427,855 and 5,391,622.
Although the methods disclosed in these publications are said to provide some advancement in the area of electrically conductive polymers, these methods include various processing difficulties. For example, one significant difficulty relates to the tendency of the blends to form highly heterogeneous two-phase systems. The high degree of phase separation is a result of the relatively small enthalpy of mixing typically associated with macromolecular systems that limits the level of molecular intermixing needed to alter the physical properties of each of the components of the blends. Accordingly, conducting polymer blends that exhibit both high electrical conductivity and good mechanical properties are very limited. In addition, conventional blending methods typically encounter the existence of a sharp threshold, known as “percolation” threshold, which is the lowest concentration of conducting particles needed to form continuous conducting chains when incorporated into another material. The percolation threshold for conductivity of the blends is met at about 16% volume fraction of the conducting polymer. This threshold is described in detail in Synthetic Metals, 22(1), 79, (1987) and the references cited therein. Due, in part, to the “percolation” effect, it is difficult to tailor the moderate electrical conductivity for a variety of uses that include the dissipation of static charge.
The second approach to improve the processability and mechanical properties of electrically conductive polymers is through the synthesis of block copolymers. Block copolymers are typically formed from the reaction of conducting polymers and conventional polymers (i.e. structural polymers such as polystyrenes, polyacrylates, polyurethanes, and the like),the product of which exhibit a combination of the properties of their segment polymers. Accordingly, segment polymers can be chosen to form copolymer products having attractive mechanical properties. Furthermore, the covalent linkage between the polymer segments allows phase separation to be limited at the molecular level, thereby providing a more homogeneous product relative to polymer blends.
Although the advantages of block copolymers over polymer blends have long been recognized, it has been found that incorporating the conducting polymer segments into block copolymers is difficult. Intrinsic electrically conducting polymers consist of a backbone of repeating units with π conjugation that limits their formation by conventional polymerization methods, such as radical polymerization, ionic polymerization or ring opening polymerization. Therefore, methods to incorporate electrically conducting polymers with other polymers are limited, and typically include linkage of short conjugated segments by flexible spacers to multi-block polymers. These previously reported block copolymers have not shown good electrical properties or nanophase separation morphology due to the short π conjugation. Recently, there have been a number of attempts to synthesize block copolymers that exhibit a nanophase separation morphology. Synthetic Metals, 41–43, 955 (1991); Nature, 369, 387 (1994); Synthetic Metals, 69, 463 (1995); Science, 279, 1903 (1998); Macromolecules, 29, 7396 (1996); Macromolecules, 32, 3034 (1999); J. Am. Chem. Soc., 122, 6855 (2001); J. Am. Chem. Soc., 120, 2798 (1998). However, few of the synthesized block copolymers have been found to exhibit good electrical properties, such as conductivity. Moreover, the processes employed to synthesize these polymers include tedious step-by-step organic synthesis to build the block copolymers, or they lack diversity in the types of copolymers available.
The discovery of additional applications and new technologies for conductive block copolymers is subject, in large part, to molecular designers ability to control the structure, properties, and function of their chemical synthesis. Those in the art have come to recognize that structure plays an important, if not critical role, in determining the physical properties of conducting polymers. PTs represent a class of conducting polymers that are thought to have the potential for furthering the advancement of new and improved applications for conductive block copolymers.
Because of its asymmetrical structure, the polymerization of 3-substituted thiophenes produces a mixture of PT structures containing three possible regiochemical linkages between repeat units. The three orientations available when two thiophene rings are joined are the 2,2′, 2,5′, and 5,5′ couplings. When application as a conducting polymer is desired, the 2,2′ (or head-to-head) coupling and the 5,5′ (or tail-to-tail) coupling, referred to as regiorandom couplings, are considered to be defects in the polymer structure because they cause a sterically driven twist of thiophene rings that disrupt conjugation, produce an amorphous structure, and prevent ideal solid state packing, thus diminishing electronic and photonic properties. The steric crowding of the solubilizing groups in the 3 position leads to loss of planarity and less π overlap. In contrast, the 2,5′ (or head-to-tail (HT) coupled) regioregular PTs can access a low energy planar conformation, leading to highly conjugated polymers that provide flat, stacking macromolecular structures that can self-assemble, providing efficient interchain and intrachain conductivity pathways. The electronic and photonic properties of the regioregular materials are maximized.
HT-poly-alkylthiopenes (HT-PATs) are conjugated polymers in which the alkylthiophene rings are connected in the head-to-tail fashion. One of the inventors of the present invention developed the first regioselective synthesis of this class of polymer (R. D. McCullough et al., J. Am. Chem. Soc., 113, 4910 (1993), and references cited therein) and a method to synthesize these polymers on a large scale (U.S. Pat. No. 6,166,172), which are incorporated by reference herein in their entirety. The “defect—free” conjugation in these polymer chains leads to better π—π between-chain overlap and give rise to highly ordered, conducting polymer structures in solid state films. This solid state structural order allows charges to travel freely without being trapped or retarded by defects. Therefore, regioregular HT-PAT films have much higher conductivity than their regiorandom analogs. In fact, HT-PATs represent one of the classes of polymers with the highest electrical conductivity.
Although the McCullough methods have made important strides in the formation of electrically conductive block copolymers, certain deficiencies exist in their application and the resultant products formed therefrom. Most significantly, it has been difficult, if not impossible, to predict with any degree of certainty the type and amount of specific end groups, such as H/Br, H/H, and Br/Br, that are produced through application of these methods. Because the end group formation of HT-PAT product is random, the reaction typically forms products mainly having amounts of either the H/H or H/Br end groups that range from 35% to 65% by weight, but are typically nearly evenly divided in amounts ranging from 45% to 55% by weight. As a result, these methods do not provide HT-PATs with well-defined, specific end-groups, inhibiting functionalization of the defined HT-PATs and incorporation of end group functionalized HT-PATs into block-copolymers with structural polymers, such as polystyrenes, polyacrylates, polyurethanes, and the like.
Accordingly, the need exists for HT-PATs and electrically conducting block copolymers formed therefrom that exhibit, or can be synthesized to exhibit, characteristics of the electronic and optical properties of semiconductors and metals and mechanical properties and processing advantages of typical plastics, and their methods of manufacture. Furthermore, new methods for preparing HT-PATs and block copolymers formed therefrom are needed that are efficient, economical, provide end group control, and produce novel block copolymers containing HT-PAT conductive segments that have both high electrical conductivity and excellent mechanical properties.